1. Field of the Invention
The present invention relates to an engine speed control method and controller therefor for controlling the idling engine speed of an engine in accordance with the loaded state of an electric load device.
2. Description of the Prior Art
For example, the Japanese Patent Publication No. 69973/1993 discloses a conventional controller for controlling the speed of an engine in accordance with an electric load. The controller uses an idling-engine-speed feedback control method while a plurality of electric load devices is connected to control the idling speed of an engine correspondingly to the on/off state of the above electric load devices. Particularly, when a plurality of electric load devices is turned on, the idling speed of an engine is controlled by changing intake air quantities to be taken into an engine through the technique for adding a predetermined electric-load correction value to each load.
FIGS. 13(a) to 13(e) are illustrations showing examples of temporal changes of an idling-engine-speed control air quantity Q.sub.ISC and an actual speed Ne when an electric load value (driving duty value) is increased by operating a radiator fan serving as an electric load device to be duty-driven under idling. The idling-engine-speed control air quantity Q.sub.ISC is an engine intake air quantity used to control the engine speed under idling. As an electric load value (driving duty value) increases since the input time t.sub.1 of an electric load value, the idling-engine-speed control air quantity Q.sub.ISC is obtained as a value obtained by further adding a predetermined idling-up correction value Q.sub.ELS while duty-driving the radiator fan to the sum of a basic air quantity Q.sub.BASE and an engine-speed feedback correction value Q.sub.NFB.
In FIGS. 13(a) to 13(e), when the idling-up correction value Q.sub.ELS is set to a predetermined value, for example, a driving duty value at a radiator-fan driving duty of 50% but an actual duty output is 90%, the idling-engine-speed control air quantity Q.sub.ISC cannot be completely corrected and the actual engine speed Ne suddenly decreases from time t.sub.1 as shown in FIG. 13(e). Thereafter, as the engine-speed feedback correction value Q.sub.NFB is increased due to engine-speed feedback correction, the actual engine speed Ne increases, slowly approaches and converges to a target engine speed Nt, and shifts to a stable state.
Then, when the above electric load is released at time t.sub.2 (t.sub.2 n&gt;t.sub.1), the idling-up correction value Q.sub.ELS under duty-driving of radiator fan added at time t.sub.1 is subtracted. However, because of increase of the engine-speed feedback correction value Q.sub.NFB due to decrease of the actual engine speed Ne between time t.sub.1 and time t.sub.2, an engine-speed feedback correction value Q.sub.NFB2 at time t.sub.2 becomes larger than an engine-speed feedback correction value Q.sub.NFB1 at time t.sub.1 and during the period for returning the increased value to the original value, the engine speed Ne increases for a while as shown in FIG. 13(e). Thereafter, the engine speed N.sub.e is shifted to a stable idling state according to engine-speed feedback correction.
Driving duties while driving a radiator fan normally change between 0 and 100%. In the case of a conventional engine-speed control method, however, the loaded state of an electric load device duty-driven is detected only under on/off state. Therefore, even if a duty output is 10 or 90%, the idling-up correction value Q.sub.ELS equal to a load value (driving duty value) under duty driving is added. Therefore, it is impossible to supply a proper electric-load correction value corresponding to an actual load value (driving duty value). That is, in the case of an electric load device to be duty-driven such as a radiator fan, though electric load values are changed correspondingly to change of driving duties, it is only possible to detect an electric load under duty driving similarly to the case in which the electric load device is turned on. Therefore, there are problems that the same electric load correction value is added independently of the electric load value is added and thus, only a correction with excess or deficiency can be performed and therefore, the actual engine speed under idling excessively increases or decreases.